In recent years, there have been used ceramic filters to selectively collect only a specific component from a mixture (mixed fluid) of many components. Since a ceramic filter is excellent in mechanical strength, durability, corrosion resistance, and the like in comparison with filters of organic polymers, the ceramic filter is preferably used for removing suspended solids, bacteria, dust, and the like in liquid or gas in wide fields such as water treatment, exhaust gas treatment, medicine, food, and the like.
In order to improve water permeation performance while securing separation performance in such a filter made of ceramic (referred to as a ceramic filter), it is necessary to increase the membrane area (area of the separation membrane). For this purpose, it is desirable that the filter has a monolith shape (or honeycomb shape). A monolith type filter is superior to a tube type filter in that it is hardly broken and that the cost is reduced. Most monolith type filters have a circular columnar external shape provided with a porous support having a large number of parallel channels (referred to as cells) therein formed in the axial direction and a separation membrane having a small pore diameter in comparison with that of the porous support on internal wall faces forming the cells.
Examples of prior art documents disclosing a conventional monolith type (or honeycomb type) ceramic filter include Patent Documents 1 to 5.
In the ceramic honeycomb filter disclosed in Patent Document 1, a slit-shaped gap portion is provided in the longitudinal direction of the porous support (porous substrate) to increase the permeation amount from the cells near the central portions of the porous support. Since, in a large monolith type filter, permeated fluid permeated through the cells near the central portion of the porous support receives large flow resistance upon flowing out from the porous support, and therefore the separation performance (filtration treatment performance) is low in comparison with the area of the filtration membrane actually formed, this ceramic honeycomb filter has been proposed for the purpose of improvement of the separation performance.
In the ceramic honeycomb filter disclosed in Patent Document 2, a filtration membrane having a smaller pore diameter than that of the porous body is formed on the inner wall faces of the numerous parallel cells formed in the longitudinal direction of the cylindrical porous body, a discharge channel is provided in the longitudinal direction of the porous body in order to increase the permeation amount from the cells near the center of the porous body, and the edge portion of the water collection cells communicating with the discharge channel is plugged by a plugging member.
In a ceramic honeycomb filter disclosed in Patent Document 3, plugging members are provided only in the predetermined spaces lest liquid stagnation should be formed. Since stagnation of the permeated fluid (filtration fluid) inside the filter can effectively be inhibited by the use of this ceramic honeycomb filter, it is possible to supply permeated fluid having high cleanliness.
The cross flow type filtration apparatus disclosed in Patent Document 4 is provided with a structure (ceramic filter) having plural (three or more) filtration cells between water collection cells and has low pressure loss, and therefore suitable for various uses for precise filtration, ultrafiltration, reverse permeation, gas separation, pervaporation, and the like.
The ceramic filter disclosed in Patent Document 5 has a structure where a filtration cell is adjacent to a side of a water collection cell and has high water permeation amount and high reverse wash efficiency when it is used for ultrafiltration or precise filtration.